1. Field of the Invention
This invention relates generally to the field of spectroscopy, and more particularly to a compact planar optic sensor capable of providing spectroscopic measurements of the complete optical absorbance spectrum of substances in the visible, ultraviolet, and/or infrared range.
2. Description of the Related Art
Spectroscopy is the measurement of the mount of light, or other radiant energy, transmitted, absorbed, or emitted by a sample of matter, as a function of the frequency or wavelength of the radiation. Either the absorption or the emission spectra can provide information on the atomic or molecular structure of a sample. For a general discussion of spectroscopy see Fundamentals of Optics, Francis A. Jenkins, Harvey E. White, McGraw Hill 1976, hereby incorporated by reference.
Infrared spectroscopy is the study of vibrational transitions. Its qualitative application is to the identification of species by interpreting their infrared fingerprint, i.e., their characteristic vibrational absorption spectrum. This is in contrast to other spectroscopic techniques developed to determine emission spectra.
Spectrometers have been used for many years as analytical instruments. Many infrared spectrometers are based on well known interferometer schemes as described in Fundamentals of Optics, and are widely available commercially. These instruments consist of a source of infrared light, emitting radiation throughout the whole frequency range of the instrument. This light is split into two beams of equal intensity, and one beam is arranged to pass through the sample to be examined. If the frequency of a vibration of the sample molecule falls within the range of the instrument, the molecule may absorb energy of this frequency, or wavelength, from the light. The spectrum is derived by comparing the intensity of the two beams after one has passed through the sample to be examined.
The Mach-Zehnder interferometer is one of many used in this well-established field. In a Mach-Zehnder interferometer, a single-mode beam is split into physically separate signal and reference branches that are subsequently rejoined to create an interference signal. For a Mach Zehnder interferometer, the difference in the optical path length of the signal and reference circuits is given by N.sub.seff L.sub.s -N.sub.reff L.sub.r where N.sub.seff is the effective index of refraction of the signal circuit, L.sub.s is the physical path length of the signal circuit, N.sub.reff is the effective index of refraction of the reference circuit and L.sub.s is the physical path length of the reference circuit. Since these four parameters can be varied independently, both refractive index effects and physical path length effects can be sensed with Mach-Zehnder devices.
The output of an interferometer, as a function of time, is the Fourier transform of the light source spectrum, as a function of frequency. This principle forms the basis of the well-known infrared Fourier transform spectrometer. In a typical arrangement, light from a continuum source passes through an absorption cell containing the molecule of interest, and then through an interferometer. The interferometer's output intensity, as measured by an appropriate detector, is digitized then transferred to a computer, which calculates the Fourier transform of the data to produce the spectrum. Because of this feature the technique is generally referred to as Fourier transform spectroscopy.
However, the infrared Fourier spectrometers in use today have certain drawbacks that render them less useful than they might be as sensors or in field applications. A typical spectrometer is generally a large and expensive precision laboratory instrument. Further, the sample to be tested must be brought into the laboratory. Thus, while infrared Fourier spectroscopy provides an ideal method of material identification, sample presentation, large size and cost of manufacture of the spectrometer discourage the application of this technology outside the laboratory setting.
It would be highly desirable to apply a compact, inexpensive, easy to use spectroscopic instrument to sense and measure, for example, low level chemical concentrations in a wide variety of samples in situ.
Several small spectroscopic instruments for measuring electromagnetic radiation are known. The reduction in the size of these instruments has been achieved using known planar waveguide technology. This technology provides the ability to generate optical systems in a chip format somewhat analogous to integrated electronic circuits. It offers the advantage of small size and mechanical stability, and allows the employment of cost favorable manufacturing methods such as photolithography. In addition, passive elements such as lenses, mirrors, beam splitters and couplers can be incorporated through the use of gratings, graded index profiles, variations in layer thickness, discrete coatings and other low production cost techniques.
One example of the use of planar waveguide technology in an optical instrument for measuring electromagnetic radiation is the Frequency Analyzer in Planar Waveguide Technology and Method of Manufacture of Auracher, et al., U.S. Pat. No. 4,548,464. Another is the Integrated Optics Spectrum Analyzer of Gregoris et al, U.S. Pat. No. 4,761,048.
Further, the Fiber Fourier Spectrometer disclosed by Ludman et al., U.S. Pat. No. 4,558,95, provides for a relatively small instrument for spectroscopic measurements of wavelengths and intensities of electromagnetic radiation from a multiple wavelength source, e.g., the flame of a rocket exhaust. It is implemented on a block of electro-optical material having beam paths formed within.
A need remains for a compact spectroscopic instrument capable of sensing, identifying and measuring the optical absorbance spectrum of substances that are not themselves sources of electromagnetic radiation.
U.S. Pat. No. 5,262,842 discloses an integrated optical interferometer for detecting substances including hydrocarbons. This patent teaches a Mach-Zehnder interferometer having a measuring arm and a comparison arm in a waveguide substrate. However, substance identification is not based on spectral analysis of the sample and is thereby limited. Because substance identification depends on the response of the selected superstrate to substance penetration, the range of substances that can be identified with a single such instrument is constrained.
A need remains for a single compact instrument capable of determining the optical absorbance spectrum of a wide variety of substances in situ.